Light flux refers to the total rate at which light is being emitted by a light source, and it may be expressed in terms such as radiant flux in units of light energy per unit of time, photon or quantum flux in units of numbers of photons per unit of time, or luminous flux in units of lumens per unit of time.
In the art of lighting using LEDs (light-emitting diodes) as light sources, various light flux setting systems exist, of which two basic types may be described as follows. One type is the analog dimming type, in which a controlling electrical level, such as a voltage, is used to adjust the current that a driver circuit puts through one or more LEDs. At a particular light flux setting the amount of current through the LEDs may be more or less steady (DC) and approximately proportional to the controlling electrical level. The light flux of the LEDs may be roughly proportional to the current through the LEDs and may thus be also roughly proportional to the controlling electrical level.
An analog dimming type of light flux setting system may take advantage of the fact that, over a certain useful current range, LEDs generate light more efficiently and last longer at lower currents than they do at higher currents. Systems that utilize highly efficient (˜85% or greater) switching converters to regulate the current through the LEDs may operate with high energy efficiency (radiant flux per electrical input power consumed) at a maximum light flux level and with even higher energy efficiency at lower light flux levels down to, for example, twenty percent of the maximum light flux level. In addition, the LEDs in such systems may, at lower light flux levels, maintain their performance over operating periods many times longer than the lifetimes that they exhibit when operating at maximum flux levels. Analog dimming may, therefore, produce energy-saving and lifetime-extending advantages in LED lighting systems operated at light flux levels substantially lower than the maximum light flux levels of which the systems are capable. Typically, a switching converter acting as an LED current driver under analog control controls the current over a five-to-one or ten-to-one range and turns the current off completely below the minimum of that range.
Another type of light flux setting system is a pulse-width-modulation (PWM) type, sometimes also referred to as a pulse-code modulation (PCM) type. This type of system sets an average light flux by allowing a rectangular-waveform signal known as the PWM signal to turn the energy source on and off repeatedly at high speed with a duty cycle ranging between zero and one-hundred percent. With LEDs, the light emission may be turned alternately fully on and fully off through modulation of the current through the LEDs by the PWM signal.
As in analog dimming, a highly efficient switching converter may be utilized to regulate the current through the LEDs. Contrary to the analog dimming approach, however, the PWM light flux setting system operates the LEDs at their maximum flux level during the part of the cycle in which the LEDs are fully on and is not designed to reduce the current to non-zero levels below the current level required for the maximum flux level. As a result, a PWM light flux setting system in the existing art generally does not take advantage of increased efficiencies that can result from lower LED currents, and the perceived lifetimes of the LEDs are increased in inverse proportion to the duty cycle, but not as much as they would be if the light flux setting were accomplished with a reduction in current as in an analog dimming system. A PWM light flux setting system may have advantages in terms of precise linear control of the light flux, which light flux may be accurately proportional to the duty cycle of the PWM signal, and in terms of stability of the wavelength spectrum of the LED, since this spectrum may have some dependence on the instantaneous current through the LED, which current is held constant during the maximum-current part of the PWM cycle. In addition, a PWM system typically can control average light flux over a much wider range than can an analog dimming system. The light flux range is limited by the minimum pulse time over which maximum current can be achieved in the driver and by the maximum period between pulses that can be allowed under flicker limitations.